The invention relates to a method and an apparatus for drying a substance that is being rapidly transported in a conveyor apparatus, in particular for drying layers of printing ink on rapidly conveyed paper. The invention relates in particular to rapidly conveyed paper with a transport velocity between 2 and 25 m/s.
When a rapidly conveyed substance is to be dried, it is extremely important that the drying operation take effect quickly. For example, the direction of movement of the material carrying a substance to be dried is changed by passage over several deflecting rollers, at a given one of which either one side or the other of the carrier material may make contact with the roller. If, for instance, in an apparatus for printing paper a layer of ink is applied to the paper and then the printed paper passes around a deflecting roller with its printed surface touching the roller, the layer of ink must be sufficiently dry before the paper reaches the roller. For other steps of the process subsequent to printing, as well, adequately dry ink is a prerequisite. Examples include the stacking of printed single pages, one over another, or the rolling up of a printed strip of paper. A similar situation is encountered in the manufacture of paper, when strips of paper that are wet throughout are rapidly conveyed for further processing.
The object of the invention is to disclose a method and an apparatus of the kind mentioned above with which drying of the substance to be dried can be accomplished quickly.
The method in accordance with the invention for drying an substance that is being rapidly conveyed in a conveyor direction, in particular for drying layers of printing ink on rapidly conveyed paper, electromagnetic radiation is directed into a drying zone so as to separate a moisture component, in particular a solvent, from the substance to be dried and the separated moisture component is transported out of the drying zone by a transport gas current. Electromagnetic radiation, in particular infrared radiation, has proved especially useful and efficient for drying. Even when the carrier material is being conveyed at high velocities, only one drying zone is needed, the length of which is short in the conveyor direction.
During separation of the moisture component from the substance to be dried, the separated moisture component can form a boundary layer that covers the substance and prevents further drying. In particular, a dynamic equilibrium becomes established at the surface of the substance to be dried, in which approximately as many moisture particles emerge from the substance as re-enter it from the boundary layer. In accordance with the invention, therefore, the separated moisture component is removed from the drying zone by a transport gas current. In particular when the transport gas is introduced continuously into the drying zone, the production of a boundary layer that would hinder drying is prevented, because the particles of the separated moisture component are transported away only a short time after they have emerged from the substance to be dried.
The electromagnetic radiation is preferably adjusted to the absorption properties of the moisture component in such a way that the radiation energy is substantially absorbed only by the moisture component and not by the remaining components of the substance to be dried and/or by a carrier material that is not moist. As a result, the moisture component is not vaporized in the strict sense, but instead the particles of the moisture component are specifically activated and expelled from the substance to be dried.
Preferably the transport gas current (D) flows into the drying zone through a region oriented transverse to the conveyor direction, from a direction that encloses an angle of 60 to 90xc2x0, preferably about 80xc2x0, with the perpendicular to the surface of the substance to be dried, thus striking the substance like a knife. As a result, the transport gas can carry along the moisture particles that emerge from the substance without transferring a substantial fraction of its kinetic energy to the substance. A mechanical deformation of the substance to be dried, which for example could cause blurring of the sharp edges of an ink imprint, is thus avoided.
Preferably the transport gas current exerts a close-range action in the region where it flows into the drying zone, inasmuch as it strikes the surface of the substance to be dried directly, so that a surface layer formed by the separated moisture component is raised away from the substance as though sliced by a knife. The sharp angle of incidence, in particular, enhances this knife-like action.
In particular the combination of the close-range effect with the orientation of the region within which the transport gas current flows into the drying zone, namely elongated in the direction perpendicular to the conveyor direction, results in an advantageously rapid drying effect over the entire extent of the region. It is also advantageous that the velocity of the transport gas current is the same over the entire width of the region occupied by the substance to be dried.
It is favourable for the transport gas current to flow along the surface of the substance to be dried either in the same direction as the substance is being conveyed or in the opposite direction, for a certain distance. This distance can in particular be longer than the length of the drying zone within which the electromagnetic radiation is incident on the substance. Thus it is ensured that the moisture particles will be transported away over the entire drying zone and even beyond it.
So as to cool the substance to be dried, if it is warmed by the electromagnetic radiation, the temperature of the transport gas current is lower, at least before it strikes the moisture component, than the temperature of the substance to be dried. This is advantageous in particular in the case of a heat-sensitive carrier material, because by the cooling of the substance to be dried, heat transfer from the substance to the carrier material can be reduced or prevented.
It is useful for the transport gas current to be formed of expanded pressurized air.
In particular if the moisture component of the substance to be dried is water, the incident electromagnetic radiation has a spectral intensity maximum in the near infrared, in particular in the wavelength range 0.8 to 2.0 xcexcm. As a result a substantial proportion of the radiation energy is introduced into the substance specifically as excitation energy for particles of the moisture component, in particular water. In the specified wavelength range there are several absorption bands of water. However, other moisture components, in particular solvents, also have absorption bands in this wavelength range.
For reasons of efficiency of the relevant thermodynamic processes, in particular to raise the overall efficacy when the method in accordance with the invention is employed, after leaving the drying zone the transport gas current flows to the source of the electromagnetic radiation in order to cool this source. In particular when the latter takes the form of a thermal radiator operated at a temperature above 2500 K, cooling is required. When the transport gas current is used in this way, it is possible either to do without another, supplementary form of cooling or such supplementary cooling means can have correspondingly smaller dimensions.
To ensure that specified temperature conditions are maintained, the temperature of the dried substance and/or the temperature of the separated moisture component and/or the temperature of the carrier material is regulated by adjusting the radiation flux density of the electromagnetic radiation incident in the drying zone according to a further development of the method. Preferably the temperature to be regulated is measured by means of a pyrometer.
It is useful to use as radiation source for the electromagnetic radiation an electric incandescent bulb, in particular a halogen bulb, and to adjust the radiation flux density by adjusting the current supply to the incandescent filament. In addition or alternatively, to adjust the radiation flux density the distance of the radiation source from the drying zone can be adjusted.
The drying is particularly efficient in a further development of the method in which components of the electromagnetic radiation that are not absorbed and thus pass through the substance to be dried are reflected back onto the substance. There the reflected radiation components are at least partially absorbed, so that the total amount of radiation absorbed is increased. Hence the radiation source employed, or the plurality of such sources, can have smaller dimensions with respect to its radiation output, or it can irradiate a larger drying zone. It is also possible to use reflected radiation components to irradiate zones along the conveyor path of the carrier material onto which no radiation that comes directly from the radiation source or sources is incident. Preferably a reflector used for reflection of the non-absorbed radiation components is cooled, in particular to minimize the emission of longer-wavelength infrared radiation.
The method in accordance with the invention can be especially advantageously employed when the carrier material is paper being conveyed at a velocity between 2 and 25 m/s. In a particular embodiment, the paper is either newsprint conveyed at a velocity between 10 and 20 m/s, in particular at about 15 m/s, or else thermoprinting paper conveyed at a velocity between 2 and 10 m/s, in particular at about 5 m/s.
In particular when thermoprinting paper is used as carrier material, the temperature of the carrier material is adjusted and/or regulated to a value below 70xc2x0 C., in particular below 50xc2x0 C. By this means an undesired thermally induced change in the carrier material or its properties can be avoided.
Preferably the transport gas current strikes the particles of the moisture component that are to be removed with a velocity between 20 and 120 m/s, and carries these particles along. In particular, the current velocity when it strikes the substance to be dried is between 30 and 40 m/s. By using a sufficiently high transport-gas velocity, for example in the specified ranges, a layer of moisture particles that have been separated from the substance to be dried, which would interfere with drying, is reliably broken up and/or lifted away from the surface of the substance, or else is never formed at all, at least not directly at the surface of the substance. In comparison with trials in which the transport gas current in accordance with the invention was omitted, drying rates 70 to 80% higher have been observed when this feature was employed.
The apparatus in accordance with the invention for drying an substance that is being conveyed rapidly in a conveyor direction, in particular for drying layers of printing ink on rapidly conveyed paper, comprises the following:
a radiation source to generate electromagnetic radiation, the radiation source being so arranged that at least part of the electromagnetic radiation is incident on the substance to be dried in a drying zone along the conveyor path of the carrier material, in order to separate from the substance a moisture component, in particular a solvent,
a transport-gas supply through which transport gas is introduced and
a transport-gas conduit that at least in parts extends transverse to the conveyor direction and is so constructed and disposed that the supplied transport gas is guided into the drying zone and strikes the substance to be dried like a knife, in order to transport the separated moisture component away from the substance to be dried.
Advantages of the apparatus in accordance with the invention have been cited above.
In particular, the transport-gas supply is a compressed-air supply and the transport-gas conduit comprises a compressed-air distributor that extends transverse to the conveyor path, in particular a distributor pipe, to distribute the compressed air flowing in from the gas inlet substantially over the entire width of the conveyor path. Preferably a single compressed-air supply, when so connected, suffices to introduce compressed air that serves to remove the moisture component from the substance to be dried over the entire width of the conveyor path.
Preferably the transport-gas conduit comprises a guide surface that runs approximately along the conveyor path of the substance to be dried and is separated therefrom by a distance that progressively diminishes in the direction of gas flow. The guide surface ends at a gas-passage gap defined by the guide surface itself and the surface of the substance to be dried. The gas enters the drying zone through this gap.
After passing through the gap the gas may, depending on the configuration of the end of the guide surface, either form eddies or continue into the drying zone in approximately laminar flow. Eddies, promoted in particular by a construction such that the end of the guide surface is tilted sharply downwards, accelerate the removal of moisture particles in the immediate vicinity of the slot but reduce the efficiency of transport at greater distances from the slot. Depending on the application, by adjusting the shape of the end of the guide surface the flow of transport gas into the drying zone can be optimized.
Especially preferred is an embodiment in which the width of the gas-passage gap is between 2 and 15 mm, in particular between 5 and 10 mm. In combination with a sharp angle of incidence of the transport-gas current onto the separated moisture component, or onto the surface of the substance to be dried, such a narrow gas-passage gap particularly enhances the knife-like action. The separated moisture particles are thus removed from the surface of the substance to be dried. In particular, the transport gas forms a partition layer, flowing either in the conveyor direction or the opposite direction over the entire length of the drying zone, between the substance to be dried and moisture particles that have already separated therefrom. In a particular embodiment, therefore, as viewed from the surface of the substance to be dried at least in the vicinity of the gas-passage gap, the moisture particles are less densely distributed close to the substance to be dried and at greater distances, whether in the transport gas current or on the other side of it, become more densely distributed. In any case the knife-like action results in a higher net emergence rate of moisture particles from the substance to be dried, i.e. it prevents appreciable diffusion of the moisture particles back into the substance. The abovementioned substantive features of the apparatus in accordance with the invention are expressly claimed as essential to the invention also for embodiments of the method in accordance with the invention.
In a preferred embodiment of the apparatus, the radiation source is an incandescent bulb, in particular a halogen incandescent bulb. Halogen bulbs can be purchased at favourable prices. Their emission temperature can be made suitable for various applications by adjusting the filament current. It is also favourable to provide reflectors at or in the region of the bulb, so that as much as possible of the emitted radiation enters the drying zone. By suitable selection of the shape and disposition of the reflectors, the spatial distribution of the radiation flux over the drying zone can also be adjusted.
Preferably a reflector is provided to reflect radiation that is not absorbed and passes through the carrier material, this reflector being disposed on the side of the conveyor away from the radiation source. In particular, a water-based cooling system is provided at this reflector.
In order to control the temperature conditions in the drying zone and the area beyond this zone in the conveyor direction, the apparatus preferably comprises a control circuit to regulate the temperature of the substance to be dried and/or the temperature of the separated moisture component and/or the temperature of the carrier material. The control circuit comprises a pyrometer to measure the temperature to be regulated, an adjustable current source that powers the incandescent bulb, and a current regulator that adjusts the current source according to the temperature value reported by the pyrometer so as to alter the current supply to the bulb appropriately.
Alternatively or in addition to the combination of adjustable current source and current regulator, the apparatus comprises a distance adjuster to vary the distance separating the radiation source from the conveyor path of the carrier material and a distance regulator, which actuates the distance adjuster according to the temperature value reported by the pyrometer so as to alter the distance of the radiation source appropriately.